Formation flight may be described as an arrangement of two or more air vehicles or aircraft flying together in a group, usually in a predetermined pattern. The benefits of formation flight may include, but are not limited to, performance advantages including aerodynamic efficiency as a result of a reduction in induced drag and fuel consumption, as well as an increase in payload and range capacity.
Flight control systems exist for controlling and maintaining multiple aircraft in a designated formation during flight. Some flight control systems are configured to enable the exchange of flight data between the aircraft being flown in formation such that the flight characteristics of each aircraft can be controlled according to the flight characteristics of the other aircraft in the formation. Generally, one aircraft in the formation is designated as a lead aircraft with the remaining aircraft being designated as trailing or wingman aircraft. According to some formation flight control systems, the flight characteristics of the trailing aircraft are controlled based on the flight characteristics of the leading aircraft. Some formation flight control systems are designed to control the flight of a trailing aircraft relative to the leading aircraft, such as for mid-air refueling events.
The formation of wake or wingtip vortices trailing behind an aircraft during flight is well known and documented. Generally, when wings are generating lift, air from below the wing is drawn around the wingtips into the region above the wings due to the lower pressure above the wing, which causes a respective vortex to trail from each wingtip. Wingtip vortices cause vortical air patterns behind the aircraft, which can affect the flight of, and be dangerous to, other aircraft and objects positioned within the wake turbulence. For example, the wingtip vortices generated by a leading aircraft may negatively affect the flight of trailing aircraft, as well as disrupting or damaging cargo being dropped by trailing aircraft. The wingtip vortices move under the influence of winds between the leading and trailing aircraft. Close-proximity formation flight systems, however, do not account for the effects of winds on the wingtip vortices because the trailing aircraft is typically close enough to the leading aircraft that the winds have not displaced the wingtip vortices.
During formation flight, some known flight control systems are equipped to estimate the position of wingtip vortices trailing a leading aircraft, and control the flight characteristics of trailing aircraft to avoid the vortices. The position of a wingtip vortex relative to a trailing aircraft is estimated based on the flight characteristics of the leading aircraft and an estimate of the wind generated by the trailing aircraft.
Further, prior systems designed to control the flight of one object relative to another object typically implemented a gradient peak-seeking approach to move the objects relative to each other to maximize or minimize a desired metric. Basically, the gradient peak-seeking approach uses a dither signal to determine a change in relative position to improve the metric. The change is effected, the results analyzed, and the position further updated once again using a dither signal to continually improve the metric.
Although conventional formation flight control systems may attempt to estimate the position of a wingtip vortex and control the position of a trailing aircraft relative to the vortex, the inaccurate estimation of the vortex position leads to inaccurate positioning of the trailing aircraft. Further, previous formation flight control systems fail to accurately track the position commands given to the trailing aircraft because such systems failed to adequately account for vortex-induced aerodynamic effects acting on the trailing aircraft. Additionally, previous formation flight control systems are not configured to prevent un-commanded movement of the trailing aircraft into a wingtip vortex due to vortex-induced air pattern disturbances and position commands. Moreover, although incremental, gradient approaches to peak-seeking may eventually position the objects close to the desired relative position, such an approach is slow, time-consuming, and less responsive.